There has been known a waste heat utilization device that is provided with a Rankine cycle device that recovers waste heat of an engine as heat energy and converts it to power and which assists the output of the engine by transmitting the output of a Rankine cycle thereof to the engine. In order to properly control the output of the engine in a vehicle equipped with such a waste heat utilization device, there is a need to accurately monitor the output torque Tr of the Rankine cycle device.
As a method of monitoring the output torque Tr of the Rankine cycle device, there is a technique disclosed in, for example, Patent Document 1. In the technique described in this Patent Document 1, the torque of an expander is estimated as the output torque Tr of the Rankine cycle device on the basis of a refrigerant pressure being a high pressure on the upstream side of the expander and a refrigerant pressure being a low pressure on the downstream side of the expander.